1. Field of the Invention
The present invention relates to a nonlinear optical element that is applicable to a light modulator, an optical switch, an optical integrated circuit, an optical computer, an optical memory, a wavelength conversion element, and a hologram element that are useful in fields such as optical information communication, optical information processing and imaging in which light is used. Furthermore, the invention relates to an organic nonlinear optical material that is necessary to manufacture the nonlinear optical element.
2. Description of the Related Art
Many functional elements such as a wavelength conversion element, a light modulator and an optical switch that are important in fields such as optical information communication, optical information processing and imaging in which light is used are embodied by using a nonlinear optical material, and particularly a secondary nonlinear optical material. Inorganic nonlinear optical materials such as lithium niobate and potassium dihydrogen phosphate have already been put to practical use and widely used as a secondary nonlinear optical material. However, organic nonlinear optical materials having superiority such as a high nonlinear optical performance, low-priced materials, low manufacturing costs, and a high manufacturability have recently received attention, and active research and development aimed at putting such materials to practical use is being carried out.
In order to exhibit a secondary nonlinear optical effect, it is essential that a center of symmetry does not exist in systems in principle, and such systems are roughly divided into systems in which an organic compound having nonlinear optical activity is crystallized into a crystal structure in the absence of a center of symmetry (hereinafter referred to as a ‘crystal system’), and systems in which an organic compound having nonlinear optical activity is dispersed in or bonded to a polymer binder and oriented by some means (hereinafter referred to as a ‘polymer system’).
It has been known that an organic nonlinear optical material in the above-mentioned crystal system is capable of exhibiting very high nonlinear optical performance. However, it is difficult to manufacture a large organic crystal that is necessary to make an element. Moreover, the organic crystal is so fragile that the organic crystal may be damaged in a process of making an element. On the other hand, preferable properties such as film-forming properties and mechanical strength that are useful for making an element are added to an organic nonlinear optical material in the above-mentioned polymer system by a polymer binder. The polymer system has thus been regarded as promising in view of a high potential for practical use.
With regard to an organic nonlinear optical material in the polymer system, it is required that an organic compound (particles) having nonlinear optical activity is uniformly dispersed or bonded without aggregating in a polymer binder and thus rendered optically homogeneous and transparent. In addition, as described above, anisotropy should be added to an organic compound having nonlinear optical activity by orienting the compound by some means in order to develop the secondary nonlinear optical effect of the resultant element. Moreover, the orientation state must be retained with stability over a long period in a temperature and humidity environment under which the element is put in order to utilize the element as a functional element.
Therefore, an organic compound having nonlinear optical activity that is used as an organic nonlinear optical material of the polymer system is required to have a low aggregating property and a superior compatibility with a polymer binder in addition to a high nonlinear optical performance. The organic nonlinear optical material of the polymer system is generally made into an element in the form of a thin film, and a wet coating method is preferably used to form the thin film. Thus, the organic compound having nonlinear optical activity that is used as the organic nonlinear optical material of the polymer system is required to have a high solubility in a solvent forcoating. Meanwhile, a polymer binder needs to have a high glass transition temperature to stably retain the orientation state of the organic compound having nonlinear optical activity that is included in the polymer binder as well as a high film-forming property and mechanical strength.
The organic compound having nonlinear optical activity needs to be oriented as described above so as to exhibit the secondary nonlinear optical activity in the organic nonlinear optical material of the polymer system, and an electric field poling method is generally used as an orienting method therefore. The electric field poling method is an orienting method in which an electric field is applied to a nonlinear optical material and then a nonlinear optically active compound is oriented in the direction of the applied electric field by Coulomb force between a dipole moment of the nonlinear optically active compound and the applied electric field. In the electric field poling method, in addition to the application of an electric field, the organic nonlinear optical material is generally heated to a temperature around a glass transition temperature thereof so as to accelerate molecular motion of the nonlinear optically active compound for supporting of the orientation.
Well-known examples of the organic compound having nonlinear optical activity include tertiary amine derivatives such as Disperse Red 1 (generally abbreviated to DR1) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (generally abbreviated to DCM) (for example, see Chemistry of Materials, 1999, Vol. 11, pp. 2554 to 2561).
Meanwhile, polymethyl methacrylate (generally abbreviated to PMMA) has been studied the most as the polymer binder. However, the glass transition temperature of PMMA is low (approximately 100° C.), and the orientation state of an organic nonlinear optical material in the polymer system that uses PMMA as the polymer binder gradually relaxes even at room temperature, whereby nonlinear optical performance considerably deteriorates over time. Thus, it has been known that, for this reason, PMMA cannot be used as the material of a practical functional element (for example, see Chemical Reviews, 1994, Vol. 94, No. 1, pp. 31 to 75).
A polymer binder to be used in place of PMMA has been actively searched for in order to solve this problem, and it has been reported that polymers having a higher glass transition temperature than PMMA such as polycarbonate, polyimide and polysulfone are effective (for example, see Japanese Patent Application Laid-Open (JP-A) No. 6-202177). However, when such a polymer having a high glass transition temperature is used as a binder, a heating temperature that is required during electric field poling must be increased. Thus, when DR1 or DCM is used as an organic compound having nonlinear optical activity in combination with the high polymer, such a compound sublimates and disappears, or is oxidized.
Further, these polymer binders having a high glass transition temperature do not necessarily have a favorable compatibility with DR1 and DCM. When DR1 or DCM is contained in a system at a high concentration in order to enhance nonlinear optical performance, the DR1 or DCM particles aggregate or crystallize. Moreover, even when DR1 or DCM is contained in a system at a low concentration, the DR1 or DCM particles aggregate or crystallize due to heating or long passage of time.
Therefore, there is a need for an organic nonlinear optical material which includes a specific organic nonlinear optical compound simultaneously having desired properties such as superior nonlinear optical performance, an amorphous property, oxidation resistance and sublimation resistance, which enables effective use of a polymer binder having a high glass transition temperature, and which has both superior nonlinear optical performance and superior stability, and a nonlinear optical element using the same.